Compositions mainly comprising a wax, such as moistureproof coatings, have been prepared mostly by mechanically mixing constituent components in their molten state (see JP-A-7-278510) or mixing a wax emulsion with other components (see JP-A-59-66598). A coating technique includes a method comprising applying a solution of a wax composition in a solvent, followed by drying to form a coating film of the wax composition (see JP-A-2002-266284). However, these techniques of the related art involve the following problems.
When a solid or high-viscous substance is mixed into a wax by mechanically mixing in a molten state, it has been difficult to uniformly disperse the components because the viscosity of the molten wax is so low that sufficient shear force is not exerted on the components to be mixed up.
In using a wax emulsion, it is impossible to disperse a component whose particle size is equal to or smaller than that of the component constituting the emulsion, which has made it difficult to obtain a uniform disperse system.
The method using a solvent involves the smell and safety problem caused by a residual solvent in an intermediate or final product. To use a large quantity of a solvent also gives adverse influences on the natural environment and the working environment. In recent years the residual solvents in lamination adhesives, printing inks, etc. used in food packaging materials have been of concern for influences on human bodies. Improvements have been awaited therefore. To address these problems, it would be a fundamental solution to establish a method using no solvent in the production steps.
Waxes having high melting points are hard to dissolve in a solvent and therefore difficult to formulate into a uniform composition. When a component to be mixed is a solvent-insoluble substance (e.g., inorganic particles), it is difficult to thoroughly disperse due to insufficient force applied to the component. Where there is a difference in specific gravity between a wax and a component being mixed, the component would settle down, resulting in uneven concentration.
The present invention has been completed in the light of the above-described problems. A first object of the present invention is to provide a wax composition mainly comprising a wax, free from residual solvent, and having a constituent component dispersed uniformly and to a process of producing such a wax composition at low cost.
A wax is widely used as a component of moistureproof agents or hot-melt adhesives. The recent increasing awareness of environmental issues has boosted the demand for wax-containing materials to have biodegradability. There are various biodegradable waxes. Having extremely low melt viscosities, waxes are usually used as mixed with resins and the like.
For example, a biodegradable hot-melt adhesive composition mainly comprising a thermoplastic resin and a tackifier is known (see JP-A-5-339557). In the composition disclosed, at least one of the thermoplastic resin and the tackifier is polylactic acid or a lactic acid copolymer, and the composition contains 50% by weight or less of a wax. A biodegradable hot-melt adhesive composition comprising raw rosin, natural rubber, and a wax is also known (see JP-A-7-278510). These compositions do not have sufficient moistureproofness. Therefore, a film prepared by melt molding the composition is unfit for use as a liner of packaging materials and containers required to have moistureproofness. While the compositions have an increased melt viscosity over that of the wax used therein in view of nature of the use as a hot-melt adhesive, their melt viscosities are not so high as to enable melt molding to produce T-die film or blown film by use of an extruder.
Among melt moldable, biodegradable compositions is the one disclosed in JP-A-2001-288295, which contains corn gluten meal and natural rubber as main components and is to exhibit high water resistance and high wet strength. The composition is, however, insufficient in moistureproof performance similarly to the hot-melt adhesive compositions disclosed in JP-A-5-339557 and JP-A-7-278510 supra.
A coating composition prepared by mixing natural rubber and carnauba wax or candelilla wax with the aid of a solvent is also known (see JP-A-2002-266284). The coating composition is used to provide a paper-made container with a moistureproof coat. Containing a large amount of a solvent, the composition has too low a viscosity to be molded by means of an extruder.
Thus, there has been no composition available that is melt moldable, sufficiently highly moistureproof, and biodegradable, nor a composition that is highly adhesive, sufficiently highly moistureproof, and biodegradable.
Accordingly, a second object of the present invention is to provide a biodegradable wax composition that can eliminate various disadvantages associated with the aforementioned known biodegradable wax compositions.
Moistureproof packaging materials or containers are manufactured by applying a wax composition containing a low molecular wax to a substrate, such as paper or a resin film, or co-extruding the wax composition together with a thermoplastic resin to obtain a multilayered laminate.
These processing steps are often carried out at or above the melting point of the low molecular wax. The problem is that the composition has poor processability because of the low viscosity of the low molecular wax. Although the viscosity of a wax composition during processing could be increased by adding a large amount of a polymer, such results in reduction of the moistureproof performance.
When products manufactured by using a wax composition, such as moistureproof containers, are handled at a high temperature for, for example, packing, sterilizing or cooking the contents, the wax layer can delaminate from the adjacent layer, or the wax layer can flow to lose its thickness uniformity, which results in reduction of the moistureproof performance.
JP-A-8-209000 describes a wax composition for providing a paper container with water resistance and the like which contains a petroleum wax having a melting point of 45° to 85° C. as a main component. This composition is difficult to melt mold by T-die extrusion or blown-film extrusion on account of its low melt viscosity. Moreover, the resulting coated paper container does not maintain the water resistance when heat treated at high temperature.
JP-A-2003-5431 discloses a toner for electrophotography which contains two waxes different in melting point. The wax to resin ratio being extremely low, the toner is not applicable to formation of a moistureproof layer.